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The body is composed of several functional units called organ systems. Each organ system is a collection of organs that function together to perform a specific job to keep the body healthy. Tissues, and the microscopic units of tissues, the cells, are the building blocks of organs. Tissues include materials such as muscles, nerves and epithelia and connective tissues that bind the other tissues together.

The cardiovascular system (cat) (dog) includes the heart and blood vessels. The cardiovascular system performs the function of pumping and carrying blood to the rest of the body. The blood contains nutrients and oxygen to provide energy to allow the cells of the body to perform work.

Thelymphatic system includes the lymph nodes and lymph vessels. The lymphatic system is part of the immune system that helps the body fight off disease. The lymphatic system also works with the cardiovascular system to return fluids that escape from the blood vessels back into the blood stream.

The digestive system (cat) (dog) includes the mouth, teeth, salivary glands, esophagus, stomach, intestine, pancreas, liver and gall bladder. The digestive system absorbs and digests food and eliminates solid wastes from the body.

The integumentary system is the skin and fur that cover the animal's body. The skin protects the underlying organs. The fur helps insulate against heat loss. Dogs and cats do not sweat through their skin. They only sweat from their footpads and nose. They lose water by panting rather than sweating.

The respiratory system (cat) (dog) includes the mouth, nose, trachea, lungs and smaller airways (bronchi and bronchioles). The respiratory system is responsible for taking in oxygen and eliminating waste gases like carbon dioxide. Because dogs and cats do not sweat through the skin, the respiratory system also plays an important role in regulation of temperature.

The urogenital system (cat) (dog) includes the kidneys, ureters, urinary bladder, urethra and the genital organs of box sexes. The urinary system is responsible for removing waste products from blood and eliminating them as urine. The genital organs are involved in reproduction.

The nervous system includes the brain, spinal cord and all the nerves that communicate between tissues and the brain and spinal cord.

The endocrine system includes several glands that produce hormones. Hormones are substances that travel through the blood stream and affect other organs. Endocrine organs include the thyroid glands, parathyroid glands, adrenal glands and part of the pancreas.

The organs of special senses (cat) (dog) allow the animal to interact with its environment; sight, taste, smell and hearing.

The hematopoietic system includes the bone marrow which is located inside the bones. Three types of blood cells are made in the bone marrow: white blood cells that fight infection, red blood cells that carry oxygen and platelets that are part of the blood clotting process.

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